linux-kernel - Re: [PATCH 1/2] x86/cpu/topology: don't write to immutable cpu_present

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Message-ID: <ZhT/ZTdRN+Gxfe8S@gmail.com>
Date: Tue, 9 Apr 2024 10:42:13 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Alexey Dobriyan <adobriyan@...il.com>, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	Jonathan.Cameron@...wei.com, linuxarm@...wei.com,
	linux@...ck-us.net
Subject: Re: [PATCH 1/2] x86/cpu/topology: don't write to immutable
 cpu_present_mask


* Thomas Gleixner <tglx@...utronix.de> wrote:

> On Sun, Apr 07 2024 at 18:26, Alexey Dobriyan wrote:
> > Workaround the following oops:
> >
> > 	topology_hotplug_apic
> > 	topo_set_cpuids
> > 		set_cpu_possible(cpu, true);
> > 		// write to __ro_after_init section after init
> 
> Duh, yes.
> 
> > adobriyan: I'm not sure what's going on, can it set unset bit here?
> > If not, then why does it repeat the job and set already set bits.
> >
> > Anyhow, let's not oops peoples' machines for now.
> 
> Adding a bandaid to paper over the non-understood real problem is
> definitely not a good plan. I take this patch as a bug report.
> 
> Proper fix below.

BTW., independently of the fix, warning about a too late set_cpu_possible() 
might still make sense - clearly it *can* be called too late by 
architecture init code :-)

It's not a performance-sensitive function in any case, so the extra 
debugging code might not hurt.

Thanks,

	Ingo